Rotating anode x-ray tube

ABSTRACT

An x-ray tube having a rotatable electrode subject to deformation when heated, and subjected to centrifugal force and means located adjacent said electrode for restraining the electrode from such deformation, said electrode and means having interfitting portions which reduce the tendency of the electrode to alter its shape when so treated.

United States Patent 11 1 [111 3,900,751

Holland et al. Aug. 19, 1975 [5 ROTATING ANODE X-RAY TUBE 3763.387 10/1973 Silbermann 313 330 3,795,832 3/1974 Holland 3l3/33O [75] Inventors: William P. Holland, West Readdlng; 18'997' 6/1974 Kaplan a a] I V I l 313/60 Robe" Azud, Rldgefield; Thomas 3 82l,58l 6/l974 Holland et al. 313 330 J. Koller, Huntington, all of Conn. [73] Assignee: The Machlett Laboratories. Inc., 'i Chatmom St f d C Allorney, Agent, or FirmHar0ld A. Murphy; John T. I Meaney; Joseph D. Pannone [22] Filed: Apr. 8, 1974 [2|] Appl. No.2 459,245 [57] ABSTRACT An x-ray tube having a rotatable electrode subject to [52] US. Cl. 313/60; 313/55; 313/330 deformation when heated, and subjected to centrifugal [5!] Int. Cl. H0lj 35/04 f r n me ns loc ed adjacent said electrode for [58] Field of Search 313/330, 55, 60 restraining the electrode from such deformation, said electrode and means having interfitting portions which [56] R f r Cit d reduce the tendency of the electrode to alter its shape UNITED STATES PATENTS when treated- 1132,442 3/l9l5 Clawson 0. 313/330 X 4 Claims, 7 Drawing Figures M Till PATENTEI] M181 9 I975 SHEET 1 [IF 3 PNENTED sum 3 of 3 ROTATING ANODE X-RAY TUBE BACKGROUND OF THE INVENTION This invention relates to electron discharge devices in general and has particular references to x-ray tubes of the rotating anode type which employ a target disc having a marginal portion movable through an electron beam which impinges upon this marginal area. or focal track. to cause generation of x-radiation therefrom.

The focal track is angled so that the impinged area or focal spot is visible from the side and so that x-rays from the focal spot may be transmitted out through the side of the envelope which encloses the anode and the. electron producing cathode.

Such targets are operated at relatively high rotational speeds such as 10,000 RPM, for example, and the bulk temperature of such targets rises to l200C or higher, with surface temperatures at the focal track reaching as high as 3000C for transient periods of time.

Targets are typically constructed with their primary bulk being molybdenum or molybdenum alloy (95% molybdenum tungsten) with the focal track having a bonded layer of tungsten or tungsten-rhenium alloy. While the cross-sectional geometry of such targets has varied somewhat, they generally comprise a disc of base material having its marginal region inclined at a predetermined angle toward the side wall of the enclosing envelope. This angled region provides an angled surface which is presented as a focal track to the electron beam. which surface may be coated with an efficient electron emitting material.

In the known state of the art. target materials and designs possess inherent problems, particularly when operated under maximum ratings. and in fact they cannot adequately fulfill the requirements of advanced diagnostic radiological procedures. These problems relate chiefly to the need for maintaining the geometric integrity 0f the target focal track angle in order to limit or reduce changes in the size or shape of the focal spot and. consequently. the ultimate x-ray output distribution.

Thus. prior art targets suffer several drawbacks which either restrict their use or seriously limit tube life expectancy when used with severe energy loads and under high-rotational speed conditions. They are susceptible to the effects of high centrifugal and thermal stresses which cause permanent geometrical distortion and dynamic imbalance and consequent reduction in x-ray film coverage.

SUMMARY OF THE INVENTION The above and other objection to and disadvantages of prior art x-ray tube rotating anode targets are overcome or reduced by the present invention wherein there is provided a target which is comprised of a disc of selected material such as tungsten, molybdenum. or a combination of rhenium. tungsten and molybdenum. which target is normally subject to large energy inputs which cause high temperatures.

In accordance with this invention there is provided a retainer or restrainer which is used with the target in such a fashion that geometrical distortion is reduced by its action with the anode. The retainer preferably is formed as a separate disc which is located closely adja cent the target and interfits therewith through comating annular ridges or hoops and grooves or channels. The ridges and channels engage in such a fashion that resistance to heat-related distortion is effected. When the target expands due to high temperature and centrifugal forces the ridges engage the channels and a suffcient resisting force results to prevent or reduce target movement or distortion in the axial direction. thereby reducing the geometrical distortion of the target.

Relatively small physical contact between the retainer and target insures that the retainer operates at a temperature less than the anode so it expands less and is mechanically stronger. The target being at a higher temperature is more susceptible to yielding under mechanical forces, and in conjunction with proper dimensions of the channel-ridge combination. the system provides sufficient control of the target forces to reduce distortion and x-ray film cutoff.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objectives of this invention are achieved by the structure shown in the drawings. wherein:

FIG. 1 is an elevational view partly in section of an x-ray tube embodying the invention;

FIG. 2 is an enlarged axial sectional view of a target employing one embodiment of this invention;

FIG. 3 is a fragmentary sectional view of a target schematically illustrating the formation of a focal spot;

FIG. 4 is a fragmentary sectional view of a portion of the target of FIG. 2 showing a modification thereof;

FIG. 5 is an axial sectional view of a target employing a different embodiment of the invention;

FIG. 6 is an enlarged sectional view of a portion of the target of FIG. 5 showing a modification thereof; and

FIG. 7 is a graph showing focal track angular deviation of a conventional target compared to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawing wherein like characters of reference designate like parts throughout the several views, there is shown in FIG. I an x-ray tube 10 of the rotating anode type embodying a preferred form of the invention. Tube I0 includes a dielectric evacuated envelope I2, preferably glass. which carries at one end a cathode structure I4 including a supporting member I6 having a transversely extending bracket I8 at one end. At the outer end of the bracket I8 there is provided a cathode head 20 which contains an electron-emitting filament as is well known in the art. Potential is supplied to the filament in the cathode head 20 from an external source by leads 22 which project from the cathode structure [4 outwardly of the tube envelope I2 substantially as shown.

In the opposite end of the envelope 12 there is located an anode structure 24 which includes a disclike target 26 which is mounted on one end of a shaft 28 between a nut 30 (FIG. 2) and a shoulder 32 on the shaft 28. Shaft 28 is rotatably mounted by means of a rotor structure 34 to which it is affixed in the usual manner of tubes of this character. Rotor structure 34 includes a rotor skirt 36 which extends parallel with and relatively closely spaced to the neck portion 38 of the envelope I2.

Envelope neck portion 38 is encircled by a suitable electrical field-inducing means whereby, when operated, it will cause the rotor and anode structure to rorate about the axis of shaft 28 in the well known manner. At the same time, filament potential is applied to the cathode to cause electrons to be emitted in a beam (FIG. 3) which is directed toward the target 26. Such impingement of the electron beam upon the target causes x-radiation to be generated at the target surface, which x-radiation will be directed outwardly through the side wall of the envelope 12.

In order to achieve this, the target 26 is made with its marginal surface adjacent the cathode head 20 inclined to a predetermined angle, such as 7l2 for example, whereby x-rays will be efficiently and properly directed through the envelope side wall, It will be understood that, as illustrated in FIG. 3, the electron beam 40 from the cathode will be focused toward the inclined surface 42 of target 26 to form a focal spot of predetermined size depending upon the focusing characteristics of the cathode. The focal spot will in elevation appear as a rectangular area. However, when projected in a direction transversely of the tube, the focal spot will appear as a square area 46 as illustrated in FIG. 3. Such a focal area may be as small as a fraction of a millimeter if desired.

in accordance with the prior art, conventional targets generally comprise a disc of tungsten, molybdenum, carbon or other selected material, which disc has a marginal portion 48 which is angled downwardly away from the cathode to provide a known symmetrical shape substantially as shown in FIGS. 2 and 3.

The angled surface 42 of such prior art target is usually coated with material having high atomic number, such as tungsten or rhcnium-tungsten which are efficient x-ray generating materials, as is well known. This coating 52 will become heated to relatively high temperatures such as about 3000C, for example, for transient periods of time when it is bombarded by electrons from the cathode during operation of the tube. Such internally generated heat will transfer into the body of the target 26 and will consequently raise the bulk temperature thereof to approximately l200C or higher. In such prior art targets, such concentrations of heat often cause damage to the targets and to the coatings, the tar get bodies often cracking and the coatings often becoming distorted or even separated from the body.

However, another serious result of such heat, coupled with the effect of centrifugal force, is the resultant warping and similar distortions of the targets which causes the focal spots to be considerably altered. For example, as shown by dotted lines in FIG. 2,the marginal portion 48 of the target will be distorted upwardly toward the cathode. Obviously this causes the projected focal area to be considerably altered or cropped. This, of course, seriously interferes with the quality of the resultant x-ray picture as is well known. Referring to FIG. 7, the line 56 therein indicates how after approximately 100 exposures a target of the prior art will distort to the extent that only -60 percent of the original focal spot coverage is utilized. After about lOO exposures, continued operation of the tube finds that focal spot coverage seriously deteriorates to an even greater extent.

In accordance with the present invention, these problems are reduced or overcome by providing the target 26 with means for physically resisting movement resulting from heat and centrifugal force. Referring particularly to FIG. 2, a retainer or restrainer 58 is located on shaft 28 immediately below the underside of the target 26, between the target and shoulder 32. if desired, a washer 60 may in turn be positioned between the retainer and shoulder, as shown.

The normal tendency of the target to distort when heated and subjected to centrifugal force is to bend upwardly as shown in dotted lines. Such bending actually starts well'inwardly from the periphery and may occur all the way to the central aperture.

To resist this distortion the adjacent surfaces of the target and retainer are provided with comating annular ridges and grooves. In the structure shown in FIG. 2, the retainer 58 is shown as being provided with two such upstanding ridges 62 which are located within respective grooves or channels 64 in the adjacent surface of the target 26. A major portion of the retainer surface nearest the target is recessed slightly as indicated at 66 so as to be constantly held out of physical contact with the target and therefore, heat will not be efficiently transferred from the target to the retainer.

However, the ridge-groove combinations are designed so that the inner sides of the ridges 62 will engage the inner side walls of the respective grooves 64. Thus, it will be understood that when the target is subjected to centrifugal and thermal expansion stresses, its tendency to distort upwardly will be resisted by the engagement between the ridge-groove surfaces, especially since the ridges remain substantially cooler than the adjacent target areas.

Referring now to FIG. 4, a modification is shown wherein the grooves 64 have outwardly directed lips 70 provided on their inner side walls, which lips 70 interfit with inwardly directed lips 72 which are provided on the inner sides of the ridges 62 on the retainer 26. This construction provides even more positive restraining effect upon the target 26.

The improvement in a structure embodying the invention is graphically illustrated in FIG. 7 wherein curve 68 indicates that target angle is considerably better with up to about 100 exposures than a target of conventional construction.

It is to be understood that the ridges 62 on the retainer 58 and the grooves 64 in the target may be respectively associated with the opposed members if desired and, furthermore, may be provided singly or in any desired quantities, although two of each are shown in FIG. 2 for illustrative purposes only. Of course, if the ridges on the target 26 are made to project downwardly into grooves in the upper surface of the retainer, contact must be made between the outer sides of the ridges and outer side walls of the grooves.

Referring more particularly to FIGS. 5 and 6, there is shown a different target structure which embodies this invention. The target 72 itself embodies an annulus having a relatively thin central portion 74 and a heavy thickened marginal portion 76. The target 72 is sandwiched between two discs 78-80 of high thermal capacity material which gathers and efficiently dissipates heat from the target 72.

Lower disc 80 rests upon a shoulder 82 provided on shaft 84 with the target 72 disposed on its upper surface as shown. Upper disc 78 rests upon the central portion 74 of the target 72 and, through nut 86 and retaining ring 88, the entire assembly is held in place on the shaft 84. Drive pins 90 on shoulder 82 interfit with aligned openings in the adjacent surface of the disc 80 to insure rotary movement of the assembly when the shaft is rotated.

In accordance with this invention annulus ridges 92 are provided on the under side of the target 72 and are disposed within aligned annular grooves 94 in the adjacent upper surface of lower disc 94 with their outer side surfaces engaging the outer side walls of the grooves 94. Thus, when the target 72 is subjected to thermal and centrifugal stresses, resultant distortion is reduced or prevented by the engagement between ridges 92 and grooves 94.

It is to be understood. of course. that the ridges 92 and grooves 94 may be provided with interlocking lips 96-98 respectively. if desired. as shown in FIG. 6 for more positive restraint. Also. the disc 80 may be provided with the ridges while the target may have the grooves. if desired. in which case the inner surfaces of respective ridge and groove combinations will be disposed in contact with one another.

In any case. disc 80 is considered as a retainer similar to retainer 58 in FIG. 2 insofar as this invention is concerned. since by virtue of the intcrfitting grooves and ridges it performs adequately to restrain movement of the target when subjected to thermal and centrifugal stresses.

It will be understood that all of the objectives of this invention have been achieved by the structures shown and described. it will be further understood that modifications and changes in the structures shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims.

Therefore all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

We claim:

I. An x-ray tube comprising an evacuated envelope containing an anode including a target, means for rotating said target about a perpendicular axis. cathode means spaced from said anode for generating a beam of electrons and directing same onto one side of said target, and retainer means engaging said target and physically restraining it from deformation resulting from heat generated by impingement of electrons thereon.

said retainer means comprising a member disposed adjacent the side of the target opposite the cathode means, the adjacent surfaces of said target and member having interfitting means for physically re- 5 straining the target from deformation when thermally stressed.

said interfitting means comprising at least one ridge on one of said adjacent surfaces and at least one aligned groove in the other of said adjacent surfaces. said ridge residing within said groove. said ridge and groove being provided with interlocking means.

2. An x-ray tube as set forth in claim I wherein said interlocking means comprises a slot provided throughout the length of the groove in a side wall thereof, and an inwardly directed rim on said ridge shaped to interfit within said slot.

3. An x-ray tube comprising an evacuated envelope containing an anode including a target. means for rotating said anode about a perpendicular axis, cathode means spaced from said anode for generating a beam of electrons and directing same onto one side of the target, and supporting members disposed on opposite sides of said target, the member on the side of the target opposite the cathode means having restraining means engaging the target and physically restraining it from deformation resulting from heat generated by impingement of electrons thereon, said restraining means comprising interfitting means on the adjacent surfaces of the target and member for effecting said restraining of the target. said interfitting means comprising at least one ridge on one of said adjacent surfaces and at least one aligned groove in the other of said adjacent surfaces, said ridge residing within said groove. said ridge and groove being provided with interlocking means.

4. An x-ray tube as set forth in claim 3 wherein said interlocking means comprises a slot provided throughout the length of the groove in a side wall thereof. and an inwardly directed rim on said ridge shaped to interfit within said slot. 

1. An x-ray tube comprising an evacuated envelope containing an anode including a target, means for rotating said target about a perpendicular axis, cathode means spaced from said anode for generating a beam of electrons and directing same onto one side of said target, and retainer means engaging said target and physically restraining it from deformation resulting from heat generated by impingement of electrons thereon, said retainer means comprising a member disposed adjacent the side of the target opposite the cathode means, the adjacent surfaces of said target and member having interfitting means for physically restraining the target from deformation when thermally stressed, said interfitting means comprising at least one ridge on one of said adjacent surfaces and at least one aligned groove in the other of said adjacent surfaces, said ridge residing within said groove, said ridge and groove being provided with interlocking means.
 2. An x-ray tube as set forth in claim 1 wherein said interlocking means comprises a slot provided throughout the length of the groove in a side wall thereof, and an inwardly directed rim on said ridge shaped to interfit within said slot.
 3. An x-ray tube comprising an evacuated envelope containing an anode including a target, means for rotating said anode about a perpendicular axis, cathode means spaced from said anode for generating a beam of electrons and directing same onto one side of the target, and supporting members disposed on opposite sides of said target, the member on the side of the target opposite the cathode means having restraining means engaging the target and physically restraining it from deformation resulting from heat generated by impingement of electrons thereon, said restraining means comprising interfitting means on the adjacent surfaces of the target and member for effecting said restraining of the target, said interfitting means comprising at least one ridge on one of said adjacent surfaces and at least one aligned groove in the other of said adjacent surfaces, said ridge residing within said groove, said ridge and groove being provided with interlocking means.
 4. An x-ray tube as set forth in claim 3 wherein said interlocking means comprises a slot provided throughout the length of the groove in a side wall thereof, and an inwardly directed rim on said ridge shaped to interfit within said slot. 